At present, the widely applied open-phase protection for circuit breakers on site is generally composed of three-phase inconsistent contact, phase current, zero-sequence current and negative-sequence current criteria. Under the working condition of operation under light load, owing to the small phase current, the negative-sequence current is quite small when the circuit breaker is in an open phase, it is difficult for the negative-sequence current to exceed its preset value, and the open-phase protection for the circuit breaker can't activate. The circuit breaker at the generator side is generally three-phase linked, not provided with three-phase inconsistent contact and unable to employ the traditional open-phase protection. The open-phase accidents caused by the breakage of tie bar in one phase of the circuit breaker at the generator side have occurred many times on site. Owing to the lack of open-phase protection, the power unit has to endure the negative-sequence current for a long time which threatens the safety of the generator. Thus, an open-phase protection independent from three-phase inconsistent contact is needed. In consideration of the above shortcomings existing in the traditional open-phase protection for circuit breakers, it is necessary to undertake research into a method and apparatus for identifying the open phase of circuit breaker independent from current magnitude and three-phase inconsistent contact.